Whenever surfaces of structural members are placed in contact with each other and subjected to the slightest relative movement, a special form of corrosion takes place. This corrosion makes itself evident by the appearance of irregular patches of black oxide which will occur even when there is very low clamp-up pressure between the members and within the first few cycles of movement. This process is known as fretting corrosion and, if permitted to continue, usually results in a premature fatigue failure of one or both of the structural members. Observations of many fatigue test specimens indicate that fretting damage accounts for at least 30% of the failures and stress corrosion cracks that have been reported on aircraft in service before 50% of the design fatigue life thereof has been expended.
In aircraft structures, fretting fatigue has become the number one failure mode superceding stress corrosion and metal fatigue due to stress concentrations. Interference fasteners and stress coining as shown in U.S. Pat. No. 3,434,327 have reduced stress concentrations about holes in structural elements, whereas the use of overage heat treatment of 7075 T6 to T73 aluminum has reduced stress corrosion in the short transverse grain direction in aluminum. Both of these improvements have been cancelled out by fretting fatigue which is the third and last of the three major failure modes for highly stressed aircraft structural materials such as steel, aluminum and titanium. Many types of coatings, tapes and rub strips have been used to reduce fretting in aircraft structures. They have added cost and weight and are unsatisfactory as they extrude out of a splice joint which reduces the clamp-up of the fastener holding the joint together to cause stress fatigue failure.
Large commercial aircraft now are designed to the latest state-of-the-art by adding material to reduce the stress level and in turn stress concentration factors at the fastener holes. As the service life of these aircraft continues, it is becoming apparent that the added weight increases fretting inside fastener holes and at the faying surfaces of splice joints. Therefore, there has been a need to provide means to prevent or greatly reduce this failure mode so that the expected lifetime of aircraft structures can be extended substantially.